1. Field of the Invention
This invention is directed to a method and device for extracting moieties, and more particularly, this invention is related to a centrifugal contactor designed to increase the residence time of liquids within the contactor to facilitate more efficient extraction of target moieties.
2. Background of the Invention
Separation of liquids can be done in a variety of ways. If the liquids are immiscible and one liquid is denser than the other, centrifugal separation is a relatively quick and simple way to mix and separate the liquids. For example, water and oil are immiscible, and water is more dense than oil. Thus, after mixing, the water and oil can be separated using a device, such as an annular centrifugal contactor.
Annular centrifugal contactors spin liquids within its rotor at a high rate to impart centrifugal forces on the liquids. Centrifugal force is inertial in nature such that for different density objects rotating in the same reference frame and at the same rate, denser objects will experience greater outward force. For example, when separating an oil-water mixture, the water will experience a greater outward force, and a separation gradient will develop between the water and oil. Using this separation gradient, the water and oil can be selectively removed from the contactor. Such technology is used to recover crude oil from sea water after oil spills.
Another application for annular centrifugal contactors and immiscible liquids is solvent extraction (a type of liquid-liquid extraction). Solvent extraction processes isolate a desired compound or compounds from a feed solution (the solution comprising the desired compound dissolved in a solvent) by mixing the solution with a second immiscible liquid and then separating the two liquids. The second liquid is chosen for the desired compound's stronger affinity for that liquid over the original solvent. Depending on the polarity of the original feed solvent, the second liquid is either a polar (e.g., aqueous) liquid, such as an acid, or a nonpolar (i.e. nonaqueous or organic) liquid. Therefore, mixing the two liquids will allow the desired compound to transfer across the phase interface into the other liquid, while undesired compounds remain in the original liquid. Upon separation, the desired compound will be isolated in one of the immiscible liquids.
Solvent extraction facilitates separation of lanthanides and actinides and also recovery of uranium and transuranics from nuclear waste. Some solvent extraction processes are done in a centrifugal contactor. However, state of the art annular centrifugal contactors are only able to extract elements that have fast kinetics, i.e., elements that will quickly transfer between the aqueous and non-aqueous (organic) phase. Fast kinetics is required because state of the art annular centrifugal contactors can only mix and hold the liquids for short residence times, typically in the range of three to five seconds for high-throughput contactors at nominal operating conditions. For most applications, the short residence time is seen as a benefit because longer residence times can cause solution degradation. For example, when reprocessing radioactive waste, solution degradation can result from irradiation of the solution. Additionally, strong acids are often used in solvent extractions, and acids can degrade the organic phase.
Notwithstanding the foregoing, short residence times are unsuitable for kinetically challenged separations that may require up to thirty seconds or more of residence time for the desired compounds to transfer between liquid phases.
Thus, a need exits in the art for an annular centrifugal contactor with increased residence times such that solvent extraction processes can be performed in extraction systems targeting solutes with slow liquid-liquid interfacial transfer kinetics.